1. Field of the Invention
The present invention relates to a mask pattern generating method, and particularly to a mask pattern generating method for generating a mask pattern for a Levenson phase shift mask used when a conductive layer having gate electrodes is patterned by photolithography.
2. Description of the Related Art
When a semiconductor device is manufactured, a fine pattern is formed on a wafer by photolithography.
In this case, first, a photoresist film of a photosensitive material is formed on a surface of a fabricated film formed on the wafer. Thereafter a photomask having a mask pattern formed therein is illuminated, whereby a mask pattern image produced by the illumination is transferred to the photoresist film, and thus light exposure is performed. The resist film to which the mask pattern is transferred is thereafter developed to form a photoresist mask over the wafer. Then, the fabricated film is etched using the photoresist mask, whereby the pattern is formed.
In this lithography technology, a fine pattern is demanded to be formed at a high resolution in order to meet demands for a higher degree of integration of semiconductor devices and higher operating speed.
As a method for making a fine pattern, a multiple exposure method using a Levenson phase shift mask as a photomask is employed (see Japanese Patent Laid-Open No. 2002-351047, Japanese Patent Laid-Open No. 2005-201967, Japanese Patent Laid-Open No. 2005-227666 and Japanese Patent Laid-Open No. 2000-258892, for example).
The Levenson phase shift mask is referred to as an alternating phase arrangement type. The Levenson phase shift mask has a plurality of line-shaped phase shifters successively arranged so that transmitted light is alternately inverted in phase. The phase shifters are formed as a mask pattern by trenching a mask substrate made of quartz, for example.
In the multiple exposure method using this Levenson phase shift mask, a shifter pattern image transfer process and a trim pattern image transfer process are performed. In the shifter pattern image transfer process, a shifter pattern image produced by irradiating the Levenson phase shift mask having phase shifters formed therein as a mask pattern with light is transferred to a photoresist film. On the other hand, in the trim pattern image transfer process, a trim pattern image produced by irradiating a trim mask, which is a photomask other than the Levenson phase shift mask and has a trim pattern formed therein, with light is further transferred to the photoresist film.
This multiple exposure method has been put to practical use to form a conductive layer such as a gate wiring layer including gate electrodes in a ULSI or the like. In the conductive layer, parts made to function as the gate electrodes need to be patterned with a fine width. For this, the Levenson phase shift mask is used in which a plurality of phase shifters are arranged so as to correspond to the parts forming the gate electrodes.
FIGS. 13A, 13B, and 13C are plan views showing the conductive layer including the gate electrodes, and the Levenson phase shift mask and the trim mask used to form the conductive layer.
FIG. 13A is a plan view showing the conductive layer 203. FIG. 13B is a plan view showing the Levenson phase shift mask used to form the conductive layer 203 of FIG. 13A. In FIG. 13B, a hatched region is a light shielding part 204 of the Levenson phase shift mask, and regions other than the hatched region are phase shifters 205a and 205b, which transmit light. FIG. 13C is a plan view showing the trim mask used to form the conductive layer 203 of FIG. 13A. In FIG. 13C, a hatched region is a light shielding part 301 of the trim mask, and a region other than the hatched region is a light transmitting part 302.
As shown in FIG. 13A, the conductive layer 203 is formed on a wafer having an active region 201 formed therein. The conductive layer 203 is formed of polysilicon, for example. In the conductive layer 203, parts corresponding to the active region 201 are formed in the shape of lines, and function as gate electrodes 203g. In the active region 201, regions facing the gate electrodes 203g function as channel regions. In the conductive layer 203, a gate contact (not shown) is formed at parts formed on a region other than the active region 201. In order to reduce wiring resistance and facilitate pattern formation, the parts formed on the region other than the active region 201 are processed into line width greater than line width of the parts formed in the shape of lines in a region corresponding to the active region 201. Incidentally, parts other than the active region 201 and the conductive layer 203 are formed so as to function as a device isolation region.
As shown in FIG. 13, the Levenson phase shift mask has the light shielding part 204 and the phase shifters 205a and 205b. The plurality of phase shifters 205a and 205b are arranged so as to correspond to the gate electrodes 203g. In this case, regions for forming the gate electrodes 203g are formed by the light shielding part 204, and the phase shifters 205a and 205b are arranged in pairs such that the light shielding part 204 is interposed between the phase shifters 205a and 205b. The phase shifters 205a and 205b extend along an extending direction of the gate electrodes 203g. One pair of phase shifters 205a and 205b is formed such that the phase of light transmitted by the phase shifter 205a and the phase of light transmitted by the phase shifter 205b are inverted with respect to each other. Thus, between the pair of phase shifters 205a and 205b, pieces of diffracted light cancel each other out, and therefore the absolute value of light intensity is decreased. Hence, light exposure can be performed while the pattern between the phase shifters 205a and 205b is separated.
As shown in FIG. 13C, the trim mask has the light shielding part 301 and the light transmitting part 302. The light shielding part 301 is patterned so as to correspond to the pattern shape of the conductive layer 203.
In forming the conductive layer 203 shown in FIG. 13A, the shifter pattern image transfer process in which a shifter pattern image is transferred using the Levenson phase shift mask as shown in FIG. 13B and the trim pattern image transfer process in which a trim pattern image is transferred using the trim mask as shown in FIG. 13C are performed. In this case, a region where the light shielding part 204 of the Levenson phase shift mask and the light shielding part 301 of the trim mask overlap each other is a dark part not irradiated with exposure light. Thus, when a positive type photoresist film is subjected to multiple exposure by the shifter pattern image transfer process and the trim pattern image transfer process as described above and then developed, the photoresist film is patterned with a photoresist material remaining in the dark part. Then, a fabricated film is etched using the photoresist pattern as a mask, whereby the conductive layer 203 can be patterned as shown in FIG. 13A.